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IodineForLunch
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bromate synthesis
Madscientist and I were talking, and the discussion turned to bromates. We came up with various possible ways of preparing bromates, but we're not
sure which ones might work, and which ones won't. Here is what we've thought of:
1. React elemental bromine with nitric acid to yeild bromic acid, water, and ntirogen oxides. Then add a hydroxide to precipitate the corresponding
bromate According to madscientist, the first step would be endothermic, but I don't know what form of energy input it would require (that is, if it
works at all).
2. Electrolysis of aqueous potassium bromide. I think this is how it might be done in industry. In room-temp water (~25 degrees) KBr is 5 times as
soluble as KBrO3.
3. Oxidation of metallic bromides or hydrobromic acid by Mn2O7.
4. Oxidation of hydrobromic acid by hydrogen peroxide, then adding hydroxide to precipitate the corresponding bromate.
Any thoughts?
David Hansen
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madscientist
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The only process I have any real confidence in is electrolysis of an aqueous solution of a bromide.
Oxidation of bromine with nitric acid sounds unlikely. HBr is oxidized by hydrogen peroxide, yielding elemental bromine and water (of course, a small
amount of the bromine reacts with the water). Any non-electrolytic method of oxidizing bromine is going to be difficult to perform.
Still interesting to think about... I'd like to know more about non-electrolytic methods. I do think that a solution of HOBr (prepared by dissolving
elemental bromine in water or hydrogen peroxide) could yield HBrO3 when heated.
I weep at the sight of flaming acetic anhydride.
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IodineForLunch
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How bout this three-step reaction?
1. React elemental bromine with hydrogen peroxide to form hypobromous acid.
3Br2 + 3H2O2 --> 6HOBr
2. Heat HOBr solution to initiate decomposition of hypobromous acid into hydrobromic acid and bromic acid.
6HOBr --heat--> 4HBr + 2HBrO3
3. Add potassium hydroxide in order to precipitate potassium bromate.
2HBrO3 + 2KOH --> 2KBrO3 + 2H2O
Net reaction:
3Br2 + 3H2O2 + 2KOH --> 2KBrO3 + 4HBr + 2H2O
Increasing the amount of H2O2 used may lead to a higher bromate yield:
3Br2 + 6H2O2 + 3KOH --> 3KBrO3 + 3HBr + 6H2O
The only problem I can think of is formation of undesired potassium bromide upon addition of the hydroxide - however, potassium bromide is 4-5 times
as soluble as potassium bromate, and will likely stay in solution while the potassium bromate precipitates out.
I'd like to hear more possible methods.
David Hansen
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IodineForLunch
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However, I'm sure that PHILOU Zrealone will come along and prove us wrong! 
J/K, no harm intended.
David Hansen
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vulture
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IIRC, the order of halogens is reversed when it comes to oxygen compounds. To be short, this comes down that bromine will replace chlorine in
chlorate.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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IodineForLunch
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Or...
K2CO3 + 5H2O2 + Br2 ----> 2KBrO3 + 5H2O + CO2
I found that in Hawley's condensed chemical dictionary.
David Hansen
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BromicAcid
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You can react a hot potassium hydroxide solution with bromine to produce potassium bromate, potassium bromide, and water.
Br2 + KOH + H2O ------> KBrO3 + H2O + KBr
The KBr and KBrO3 can be seperated from each other by fractional crystalization.
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blip
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Why do you not reduce your coefficients down to the smallest possible, IodineForLunch? I must be missing something. For example:
| Quote: | | 3Br2 + 3H2O2 --> 6HOBr |
can be reduced down to Br<sub>2</sub> + H<sub>2</sub>O<sub>2</sub> <s> ></s>
2HOBr
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KABOOOM(pyrojustforfun)
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<blockquote>quote:<hr>Br2 + KOH + H2O ------> KBrO3 + H2O + KBr <hr></blockquote>that's unbalanced.
6KOH + 3Br<sub>2</sub> <s> ></s> KBrO<sub>3</sub> + 5KBr +
3H<sub>2</sub>O
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woelen
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*BUMP*
I did the synthesis of KBrO3 by means of electrolysis. There is a nice synth on www.versuchschemie.de by garage chemist:
http://www.versuchschemie.de/htopic,6656,kaliumbromat.html.
He used platinum wire as anode. I had to use a thick graphite rod as anode. I also used such a graphite rod as cathode.
I dissolved as much as possible of KBr in 35 ml of water.
I also added 20 mg of K2Cr2O7 and dissolved this.
The solution now is pale yellow.
I dipped the electrodes in the liquid and used a current of 4 A for the electrolysis (using a 13.8 V power source, with suitable series resistor in
order to regulate current and obtain a fairly constant current, independent of precise chemical composition of the electrolyte).
The evolution of hydrogen worked very well, and it also remained good for all time I did the electrolysis. The effect, described by garage chemist,
that bromate is reduced back to bromide, did not occur. I did not add a drop of soap in order to suppress spraying. For that purpose, I used a plastic
cap, with tight holes in it for the electrodes. Any spraying only wetted the bottom side of the cap and the liquid runs back into the beaker.
The liquid became very hot (probably well over 70 C, I really could not stand touching the long/tall glass in which the solution was without gloves).
I stopped electrolysis after 3 hours. After that period the liquid has turned deep green and there was some black soot on the bottom from the graphite
anode, but this was only a very little bit. There hardly is any erosion of the anode. That's good.
I decanted the hot clear liquid from the black soot and hence got rid of the black stuff, and what remains is a hot clear green liquid with a
brown/yellow tinge. On slow cooling down, fairly large crystals of KBrO3 settled. I was nicely surprised by the good yield.
Now comes the problem: My crystals are not nice white, but they are light green with a yellow/brown tinge. I think they have chromium (III) in them,
together with remains of dichromate. I carefully rinsed the crystals with cold distilled water, but that does not help. The KBrO3 is a potent
oxidizer, so that is OK, but I do not like its color, it should be white.
I could of course recrystallize, but that would introduce quite a large loss of KBrO3. Why do I get green/brown crystals and garage chemist obtains
snowwhite crystals, also directly from his first batch, without the need to recrystallize? He even used more K2Cr2O7 than I did, relative to the
amount of KBr he used. Also, his K2Cr2O7 did not become green and apparently is not reduced to chromium (III). My liquid was dark green at the end,
his still is yellow.
[Edited on 12-10-06 by woelen]
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woelen
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I did recrystallize the material and took the loss for granted. The bromate now is almost white. Only a very vague green color remains, which is only
noticeable, if the KBrO3 is put next to a purely white solid.
But still, I do not understand how the pure material can be achieved at once. I did another experiment, now with 1 A current, but the result is the
same. The solid I obtain is off-white and yellow or green, depending on how long I perform the electrolysis. I simply cannot prevent contamination of
the crystals with chromium  .
I also did a test for bromide content in my sample. With respect to bromide it is very clean. If I add a spatula of this to 2 M H2SO4, then no yellow
color at all can be observed, and no smell of bromine can be observed. As soon as I add a spatula of KBr as well, the liquid becomes turbid and rusty
brown, due to very finely dispersed droplets of Br2. After a few minutes I have a big drop of Br2 at the bottom.
So, I regard my making of KBrO3 quite succesful, the only flaw remaining some traces of Cr (III) and possibly Cr(VI).
[Edited on 13-10-06 by woelen]
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not_important
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Interesting. Are you willing to sacrifice a few grams of bromate to try crystallizing with controlled amounts of Cr(VI) or Cr(III) to see if it is
only the +3 state that is causing the problem? It may be that the carbon electrodes cause too much reduction of the hexavalent chromium.
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woelen
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I have quite some KBrO3 now, I now can make this routinely if I wish, approximately 10 grams per evening. I do not want electrolysis cells running
overnight, hence the slow progress, but it does not bother me. I only need gram-quantities.
I have an analytic balance with 0.1 mg resolution, so I can do nice precise experiments.
I can do an experiment with 500 mg of KBrO3 in 3 ml of water and 5 mg of K2Cr2O7 added, and see how yellow my crystals of KBrO3 are after
recrystallization.
With Cr(III) things become less satisfactory. I do have CrCl3.6H2O and KCr(SO4)2.12H2O, but none of these provide a good model for the situation in
the electrolysis cell. The chloride and sulfate definitely will interfere, they both strongly coordinate to chromium (III) ions, when the solution is
heated (try it, dissolve some chrome alum in water, and then heat, you'll see a color change from violet to deep green).
I'll do the experiment with KBrO3/K2Cr2O7 this weekend. If the resulting crystals are still nice white, then we know that the Cr(III) is the problem.
--------------------------------------------------------------------------
Yet another thing, could it be possible to make KBrO4 from the KBrO3? I would love to experiment with that compound. I read something about the need
of fluorine, but aren't there any other methods, which are more easily accessible for the home chemist? What would be the solubility of KBrO4, would
it be easy to isolate? If there is analogy with KClO4 then I expect it would be easy.
[Edited on 13-10-06 by woelen]
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Chemist514
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Purification of Potassium Bromate
Hello!
Not sure if this is in anyway usefull to you peeps, but its from The Purification of Laboratory Chemicals 5th edition...
Potassium bromate [7758-01-21 M 167.0, m 350°(dec at 370°), d 3.27. Crystd from distilled
H20(2mL/g) between 100° and Oo. To remove bromide contamination, a 5% soln in distilled H20, cooled to
loo, has been bubbled with gaseous chlorine for 2h, then filtered and extracted with reagent grade CCl4 until
colourless and odourless. After evaporating the aqueous phase to about half its volume, it was cooled again
slowly to about loo. The crystalline KBrO3 was separated, washed with 95% EtOH and vacuum dried [Boyd,
Cobble and Wexler J Am Chem Soc 74 237 19521. Another way to remove Br- ions was by stirring several
times in MeOH and then dried at 150° [Field and Boyd J Phys Chem 89 3767 19851.
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Chemist514
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Opps, lemme correct a couple things that got messed up...
I should read "between 100 and 0 degrees"
and "then cooled slowly to 10 degrees"
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not_important
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| Quote: | Originally posted by woelen
(snip)
Yet another thing, could it be possible to make KBrO4 from the KBrO3? I would love to experiment with that compound. I read something about the need
of fluorine, but aren't there any other methods, which are more easily accessible for the home chemist? What would be the solubility of KBrO4, would
it be easy to isolate? If there is analogy with KClO4 then I expect it would be easy.
[Edited on 13-10-06 by woelen] |
I've never read of any other method than using F2, XeF2, or possibly higher xenon fluorides. JACS 90, 1900 (1968), Inorg Chem 8, 223 (1969), and
Inorg Synthesis 13,1 (1972) are the references that I have, along with a scribbled note about some patents using F2.
This might have more, it's pay-to-see for me
http://www.turpion.org/php/paper.phtml?journal_id=rc&pap...
You could try for BrO3F, which attacks almost everything including teflon. 8-)
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woelen
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| Quote: | | You could try for BrO3F, which attacks almost everything including teflon. 8-) |
I'll first stick to the KBrO3/K2Cr2O7 recrystallisation experiment and post an update if I have results (after this weekend).
Next, I'll have a look at the attic and see if I still have a few pounds of XeF4 or BrO3F lying around. I'll post an update on this as well, but that
could take a _slightly_ longer time than next weekend  .
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Al Koholic
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You know woelen...perhaps the soap is having an effect on the precipitation of the bromate crystals. You should add a drop to your next batch to see
what happens.
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garage chemist
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I have once tried to make perbromate by electrolysis of bromate, because I read in Brauer that perbromates could be prepared in this way.
However, electrolysis of a pure NaBrO3 (NaBrO3 was from a chemical supplier) solution with platinum anode did only yield oxygen at the anode and
nothing else.
Recently I had a look into a book on inorganic chemistry (Earnshaw, Chemistry of the elements) and read that perbromate was prepared in 1% yield by
electrolysis of lithium bromate solution. So this is not a viable way to get perbromates.
The only way to obtain useable quantities of perbromate is to oxidise a strongly alkaline bromate solution with fluorine, as done in Brauer. This is
of course much cheaper than using xenon compounds, but also less convenient.
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woelen
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I did the experiment of recrystallizing KBrO3 with 1% K2CrO7. I had 660 mg KBrO3 and 6.4 mg K2Cr2O7, dissolved in appr. 3 ml hot H2O. This solution
has quite a strong yellow color. On cooling down, almost all KBrO3 separates from this solution. When it is rinsed, then it is totally white!! This
really is amazing.
I now had much more dichromate, relative to the amount of KBrO3, then in the electrolysis experiment. So, the chromium (III) must be the problem. The
color also confirms this, the solid, obtained from the electrolysis cell is light green.
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not_important
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So it would seem to be that the graphite electrodes reduce enough Cr(VI) to Cr(III) to give you problems. Going to be a problem removing it, except
possibly with ion exchange or co-precipitation.
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woelen
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I used a graphite cathode as well. Garage chemist used a copper wire. I used a graphite cathode, because I did not want any extra metal contamination
besides the chromium. But probably this was not the right decision.
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garage chemist
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I think that the graphite anode is the cause of your contamination. It gives off colloidal partivcles which can adsorb metal ions, like chromium(III)
and be impossible to remove by filtration.
A copper or iron cathode does not introduce any extra metal ions.
The metal is completely protected against corrosion by the negative charge.
I'm sure that using a platinum anode and copper cathode will solve your problem.
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woelen
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| Quote: | | I'm sure that using a platinum anode and copper cathode will solve your problem. |
Yes, I also am sure about that. I ordered some platinum wire (5 inch of 0.4 mm Pt/Ir alloy, 95/5, and 10 inch of 0.3 mm of 99.9% platinum) and when it
arrives, I'll try again. But still, the graphite anode method also is quite useful. The batch of KBrO3 I now have is VERY pale green after a single
recrystallization. Without a pure white reference (e.g. white table salt) besides it, you don't notice the color. For the great majority of
experiments, this KBrO3 is perfectly suitable. It has no dichromate in it, it only is chromium (III). I did the very sensitive acid/H2O2 test on
dichromate, and this test was negative.
I like the graphite method better, because it is accessible for more people. If I share such a method, then I like it if other people also can make
such an interesting compound without too much problems.
I'll forget about the making of KBrO4. The F2-method of course is out of reach of home chemists and other homelab methods indeed seem not to exist.
[Edited on 15-10-06 by woelen]
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Zinc
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| Quote: | Originally posted by woelen
I dissolved as much as possible of KBr in 35 ml of water.
I also added 20 mg of K2Cr2O7 and dissolved this.
The solution now is pale yellow.
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Is K2Cr2O7 required or can the electrolysis be performed with out it?
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